The present invention relates to a device for determining a measureable variable on a measurement object, and more particularly to a coordinate measuring machine.
Coordinate measuring machines are generally used for determining spatial coordinates of defined measuring points on a measurement object. They allow the determination of geometric dimensions and/or the three-dimensional shape of the measurement object by exploiting a plurality of spatial coordinates. A typical field of use is the measurement of workpieces for quality control. Typically, coordinate measuring machines have a frame structure including a measuring head that can be moved relative to the measurement object. The measuring head is brought into a defined position relative to the measuring point on the measurement object, and the spatial coordinates of the measuring point can then be determined from the spatial position of the measuring head.
It is a known problem with coordinate measuring machines that the workpiece table on which a measurement object is placed for a measurement task can be deformed under the load of the measurement object. When this deformation carries over to the frame structure, measurement results are corrupted in a way that is difficult to correct. There are various proposals for solving this problem.
German patent application DE 31 18 612 A1 proposes a machine design wherein a baseplate rests on a bed by a first plurality of machine feet. Provided in the baseplate are vertical sleeves in which a plurality of second machine feet are arranged and are supported on the bed. The second machine feet can be adjusted in height and bear on their upper, free end a workpiece table on which the measurement object can be placed. Since the frame structure with the moveable measuring head is mounted on the baseplate, deformations of the workpiece table do not act on the frame structure. In other words, the linearity of the guides of the moveable measuring head is largely independent from the weight of the measurement object. However, the known device is relatively complicated in design and installation, since a plurality of machine feet must be mounted and adjusted. Moreover, in this known device there is the risk of the baseplate being displaced together with the frame structure in relation to the tool table, and this can lead to stresses and measuring inaccuracies.
EP 0 604 380 A1 proposes a coordinate measuring machine wherein the workpiece table is supported on a baseplate via at least three feet. The baseplate also has at least three feet, with one foot of the baseplate being arranged below one foot of the workpiece table in each case.
U.S. Pat. No. 4,594,791 discloses a coordinate measuring machine of a portal design wherein the workpiece table is supported on a baseplate via feet. The portal is intended to embrace the workpiece table in an annular structure in order to achieve greater stiffness.
U.S. Pat. Nos. 4,727,653 and 4,495,703 each disclose coordinate measuring machines wherein the guides for the frame structure that bears the measuring head are arranged at the same basic body on which the measurement object is also placed. This basic body can be produced from stone. In order to achieve sufficient stiffness, very heavy basic bodies are required in the case of such a design. Nevertheless, there is a risk here of deformation under the weight of a heavy measurement object.
DE 1 798 191 A1 discloses a coordinate measuring machine having a workpiece table that can be adjusted in height. Tooth racks are provided for locking the workpiece table in a desired baseplate position. This design serves the purpose of measuring objects with sizes that deviate greatly from one another with limited travels of the measuring head.